Switch module

ABSTRACT

A switch module includes a first terminal, first and second filters, and first and second switches. Impedance of the first filter for a signal in a stop band is capacitive. When the first switch is turned OFF, impedance of the first switch is capacitive, and impedance of the first filter seen from an end portion of the first switch connected to the first filter is not in a short state and impedance of the first filter seen from the first terminal is in an open state.

This application claims priority from Japanese Patent Application No.2016-096032 filed on May 12, 2016. The content of this application isincorporated herein by reference in its entirety.

BACKGROUND

The present invention relates to a switch module that switches betweensignal paths in accordance with the frequency band.

Electronic devices that transmit and receive signals by using multiplefrequency bands are known. In such electronic devices, a switch modulethat switches between signal paths in accordance with the frequency bandmay be used. In this switch module, a signal of a certain frequency bandmay leak from a terminal other than a target output terminal. This maycause a device or a circuit connected to such a terminal to malfunction.In order to improve the performance of such a switch module, it isnecessary to enhance isolation characteristics representing the degreeof isolation between terminals.

Japanese Unexamined Patent Application Publication No. 2004-140696discloses a single-pole n-throw (SPnT) radio-frequency switch circuitthat switches between plural receive output terminals and a transmitinput terminal. In this radio-frequency switch circuit, a switch isdisposed between a receive output terminal and a receive circuit to turna radio-frequency signal ON/OFF. When a transmit signal is input from atransmit circuit, this switch is turned OFF. This configuration makes itpossible to reduce a leakage of a transmit signal into a receive circuitand to enhance isolation characteristics of the radio-frequency switchcircuit.

BRIEF SUMMARY

In the above-described configuration, in order to more reliably preventa signal leakage by turning OFF the switch, a shunt-connected switch isusually provided between a signal path and a ground point. For example,Japanese Unexamined Patent Application Publication No. 2014-93610discloses a radio-frequency switch circuit including shunt-connectedswitches that connect a signal path and a ground point. In thisradio-frequency switch circuit, the shunt-connected switches are turnedON so that the input impedance will be made to be almost 0 to causeimpedance mismatching, thereby eliminating the influence of theimpedance of a circuit connected to the radio-frequency switch circuit.In the radio-frequency switch circuit disclosed in this publication, aSPnT switch module including shunt-connected switches provided in asignal path from a common terminal P1 to input/output terminals P2through P7 is provided.

FIG. 14 is a circuit diagram of the radio-frequency switch circuit shownin FIG. 13 of this publication. As shown in FIG. 14, the shunt-connectedswitch disposed between the common terminal P1 and the input/outputterminal P6 that is electrically connected to the common terminal P1 isturned OFF. In contrast, the shunt-connected switches disposed betweenthe common terminal P1 and the input/output terminals P2 through P5 andP7 that are electrically disconnected from the common terminal P1 areturned ON. When a signal of a certain frequency band passes between thecommon terminal P1 and the input/output terminal P6, series-connectedswitches disposed on the signal paths of OFF ports are turned OFF andthe associated shunt-connected switches are turned ON. With thisconfiguration, the impedance of the input/output terminals on the OFFports seen from the common terminal P1 is not influenced by thecharacteristic impedance of the devices connected to the input/outputterminals. That is, the impedance of each input/output terminal seenfrom the common terminal P1 is not influenced by the characteristicimpedance of a device connected to the input/output terminal because ofthe effect of the associated shunt-connected switch that is turned ON,and is determined by the capacitance of the series-connected switch thatis turned OFF.

As in the switch module disclosed in this publication, a shunt-connectedswitch may be disposed on a signal path from a common terminal P1 toeach input/output terminal. In this case, in order to reduce theinsertion loss, which may occur when a certain series-connected switchis turned ON, the capacitance of another series-connected switch that isturned OFF may be decreased so that power of leakage which may occur viathis capacitance can be reduced. However, decreasing of the capacitanceof a series-connected switch that is turned OFF increases the resistanceof this series-connected switch when it is turned ON, thereby increasingthe insertion loss of this series-connected switch. In this manner, thecapacitance of a series-connected switch that is turned OFF and theresistance of this series-connected switch that is turned ON have atradeoff relationship. It is difficult to find suitable values of thecapacitance and the resistance of a series-connected switch which maycontribute to reducing the insertion loss of the overall switch module.It is thus difficult to reduce the insertion loss of a switch moduleincluding shunt-connected switches.

The present disclosure has been made in view of the above-describedbackground. The present disclosure reduces the insertion loss of aswitch module.

According to an embodiment of the present disclosure, there is provideda switch module including a first terminal, first and second filters,and first and second switches. The first filter allows a signal in afirst frequency band to pass through the first filter and stops a signalin a second frequency band from passing through the first filter. Thefirst switch switches between electrical connection and disconnectionbetween the first terminal and the first filter. The second filterallows a signal in a third frequency band to pass through the secondfilter. The third frequency band is included in the second frequencyband. The second switch switches between electrical connection anddisconnection between the first terminal and the second filter.Impedance of the first filter for a signal in the second frequency bandis capacitive. When the first switch is turned OFF, impedance of thefirst switch is capacitive, and impedance of the first filter seen froman end portion of the first switch connected to the first filter is notin a short state and impedance of the first filter seen from the firstterminal is in an open state.

The short state is a state in which impedance is as low as almost 0. Theopen state is a state in which impedance is as high as being almostinfinite.

In a switch module according to an embodiment of the present disclosure,the impedance of the first filter seen from an end portion of the firstswitch connected to the first filter is not to be made in the shortstate, thereby making it possible to reduce the insertion loss of theswitch module.

Other features, elements, characteristics and advantages of the presentdisclosure will become more apparent from the following detaileddescription of embodiments of the present disclosure with reference tothe attached drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a circuit diagram of a switch module according to a firstembodiment;

FIG. 2 is a circuit diagram of a switch module according to a firstcomparative example;

FIG. 3A is an equivalent circuit diagram of the switch module accordingto the first comparative example shown in FIG. 2;

FIG. 3B is a Smith chart representing an impedance change on a signalpath from a common terminal to an input/output terminal in the switchmodule shown in FIG. 2;

FIG. 4A is an equivalent circuit diagram of the switch module accordingto the first embodiment shown in FIG. 1;

FIG. 4B is a Smith chart representing an impedance change on a signalpath from a common terminal to an input/output terminal in the switchmodule shown in FIG. 1;

FIG. 5A is a Smith chart representing simulation results of an impedancechange of a SAW filter in the first embodiment;

FIG. 5B is a Smith chart representing simulation results of an impedancechange of a filter in a second comparative example;

FIG. 6A is an equivalent circuit diagram of a switch module according tothe second comparative example;

FIG. 6B is a Smith chart representing an impedance change on a signalpath from a common terminal to an input/output terminal in the switchmodule of the second comparative example;

FIG. 7 is a graph representing the insertion loss of the switch modulesof the first embodiment and the first and second comparative examples;

FIG. 8 is a circuit diagram of a switch module according to a firstmodified example of the first embodiment;

FIG. 9 is a circuit diagram of a switch module according to a secondmodified example of the first embodiment;

FIG. 10 is a circuit diagram of a switch module according to a secondembodiment;

FIG. 11 is a circuit diagram of a switch module according to a firstmodified example of the second embodiment;

FIG. 12 is a circuit diagram of a switch module according to a secondmodified example of the second embodiment;

FIG. 13 is a circuit diagram of a switch module according to a thirdembodiment; and

FIG. 14 is a circuit diagram shown in FIG. 13 of Japanese UnexaminedPatent Application Publication No. 2014-93610.

DETAILED DESCRIPTION

Embodiments of the present disclosure will be described below in detailwith reference to the accompanying drawings. In the drawings, the sameelements or similar elements are designated by like reference numerals,and an explanation thereof will be provided only once.

First Embodiment

FIG. 1 is a circuit diagram of a switch module 1 according to a firstembodiment. As shown in FIG. 1, the switch module 1 includes a commonterminal P1, input/output terminals P2 and P3, first and second filtersSAW1 and SAW2, which are surface acoustic wave (SAW) filters, and firstand second switches SW1 and SW2. The pass band of the second filter SAW2is included in the stop band of the first filter SAW1. The switch module1 is a single-pole double-throw (SPDT) switch module. Alternatively, theswitch module 1 may be a SPnT (n is three or greater) switch module.

The common terminal P1 corresponds to a first terminal of an embodimentof the disclosure. The pass band of the first filter SAW1 corresponds toa first frequency band of an embodiment of the disclosure, and the stopband of the first filter SAW1 corresponds to a second frequency band ofan embodiment of the disclosure. The pass band of the second filter SAW2corresponds to a third frequency band of an embodiment of thedisclosure. The pass band of the first filter SAW1 does not overlap thatof the second filter SAW2.

On a path from the common terminal P1 to the input/output terminal P2,the first switch SW1 and the first filter SAW1 are connected in thisorder. The first switch SW1 switches between electrical connection anddisconnection between the common terminal P1 and the first filter SAW1.

On a path from the common terminal P1 to the input/output terminal P3,the second switch SW2 and the second filter SAW2 are connected in thisorder. The second switch SW2 switches between electrical connection anddisconnection between the common terminal P1 and the second filter SAW2.

The first and second switches SW1 and SW2 include field effecttransistors (FETs), for example. The ON/OFF state of the first andsecond switches SW1 and SW2 is controlled by a controller (not shown).The configuration of switches, which will be discussed later, is alsosimilar to that of the first and second switches SW1 and SW2.

The impedance of the first filter SAW1 for a signal in the stop band iscapacitive. The impedance of the second filter SAW2 for a signal in thestop band is also capacitive. The impedance of SAW filters, which willbe discussed later, for a signal in the stop band is also capacitive.

In FIG. 1, the first switch SW1 is turned OFF, while the second switchSW2 is turned ON. The ON/OFF states of the first and second switches SW1and SW2 are set in this manner when, for example, signals are receivedfrom an antenna connected to the common terminal P1 and a signal in thepass band of the second filter SAW2 is output from the input/outputterminal P3.

FIG. 2 is a circuit diagram of a switch module 100 according to a firstcomparative example. The difference between the switch module 100 andthe switch module 1 is that the switch module 100 includes fourth andsixth switches SW4 and SW6 as shunt-connected switches. Theconfigurations of the other elements are similar to those of the switchmodule 1, and an explanation thereof will thus be omitted.

The fourth switch SW4 switches between electrical connection anddisconnection between a ground point and a first node C1 between thesecond switch SW2 and the second filter SAW2. The fourth switch SW4 isturned OFF when the second switch SW2 is turned ON. The fourth switchSW4 is turned ON when the second switch SW2 is turned OFF. In FIG. 2,the second switch SW2 is ON and the fourth switch SW4 is OFF.

The sixth switch SW6 switches between electrical connection anddisconnection between a ground point and a third node C3 between thefirst switch SW1 and the first filter SAW1. The sixth switch SW6 isturned OFF when the first switch SW1 is turned ON. The sixth switch SW6is turned ON when the first switch SW1 is turned OFF. In FIG. 2, thefirst switch SW1 is OFF and the sixth switch SW6 is ON.

When a signal MS1 in the pass band of the second filter SAW2 passesbetween the common terminal P1 and the input/output terminal P3, itpartially leaks to the first switch SW1 as a signal LS1 even though thefirst switch SW1 is OFF. In this case, without the sixth switch SW6, thefirst switch SW1 (OFF) does not become the ideal open state and isregarded as a capacitance, instead. Thus, the impedance seen from thethird node C is influenced by the impedance of the first filter SAW1. Toeliminate the influence of the impedance of the first filter SAW1, thesixth switch SW6 is disposed at the third node C3 for switching betweenelectrical connection and disconnection between the third node C3 and aground point. When the first switch SW1 is turned OFF, the sixth switchSW6 is turned ON so that the impedance seen from the third node C3 canbe made in the short state, thereby eliminating the influence of theimpedance of the first filter SAW1. That is, the impedance of theinput/output terminal P2 seen from the common terminal P1 is determinedby the characteristic impedance of the first switch SW1 that is turnedOFF.

In order to reduce the insertion loss, which may occur when the secondswitch SW2 is turned ON, the capacitance of the first switch SW1 that istuned OFF may be decreased so that power of leakage which may occur viathis capacitance can be reduced. To decrease the capacitance of thefirst switch SW1 that is turned OFF, it is necessary to reduce the sizeof a transistor (for example, the gate width of a FET) used in the firstswitch SW1. However, decreasing of the size of the transistor increasesthe resistance of the first switch SW1 when it is turned ON, therebyincreasing the insertion loss of the first switch SW1. In this manner,the capacitance of the first switch SW1 that is turned OFF and theresistance of the first switch SW1 that is turned ON have a tradeoffrelationship.

In the first embodiment, attention is focused on the fact that theimpedance of a series-connected switch that is turned OFF and theimpedance of a SAW filter for a signal in the stop band are bothcapacitive (the imaginary part of the impedance is negative). Then,without the use of shunt-connected switches, the impedance of the firstfilter SAW1 seen from the common terminal P1 can be represented by thecombined impedance of the impedance of the first filter SAW1 that isturned OFF and the impedance of the first filter SAW1 for a signal inthe stop band. With this configuration, the impedance of the firstfilter SAW1 seen from the common terminal P1 can be made in the openstate.

Referring back to FIG. 1, the impedance of the first switch SW1 that isturned OFF is capacitive. The impedance of the first filter SAW1 seenfrom the end portion of the first switch SW1 connected to the firstfilter SAW1 is not in the short state, and the impedance of the firstfilter SAW1 seen from the common terminal P1 is in the open state. Withthis configuration, the impedance of a signal path from the commonterminal P1 to an input/output terminal via an associated switch that isturned OFF is not in the short state, but in the open state.

A leakage of a signal into a signal path including a switch that isturned OFF becomes smaller as the impedance of this signal path iscloser to the open state. Even without a shunt-connected switch, makingthe impedance of a signal path be closer to the open state can prevent asignal leakage from a terminal connected to this signal path and thusreduce the insertion loss.

In the first embodiment, it is possible to reduce the insertion loss ofthe switch module 1.

In order to show how the impedance of the switch module 1 of the firstembodiment differs from that of the impedance module 100 of the firstcomparative example, the impedance of the switch module 100 will firstbe explained with reference to FIGS. 3A and 3B, and then, the impedanceof the switch module 1 will be explained with reference to FIGS. 4A and4B.

FIG. 3A is an equivalent circuit diagram of the switch module 100 shownin FIG. 2. FIG. 3B is a Smith chart representing an impedance change onthe signal path from the common terminal P1 to the input/output terminalP2. The reason why the equivalent circuit of the switch module 100 isrepresented by that shown in FIG. 3A is as follows.

A switch including a FET that is turned OFF stores some electric chargeand can thus be regarded as a capacitor. The impedance of a switchincluding a FET that is turned OFF is capacitive, as in a capacitor. Aswitch including a FET that is turned ON can be regarded as a very smallresistor.

By taking these points into account, the first and fourth switches SW1and SW4 that are turned OFF in FIG. 2 are represented as capacitors inFIG. 3A. The second and sixth switches SW2 and SW6 that are turned ON inFIG. 2 are represented as resistors in FIG. 3A.

SAW filters include interdigital transducer (IDT) electrodes. The combteeth of the IDT electrodes serve as capacitor electrodes, and thus, thecharacteristic impedance of the SAW filters for a signal in the stopband is capacitive. The characteristic impedance of the SAW filters fora signal in the pass band is set to be about 50Ω, for example.

In FIG. 3B, a point ZO is a point at which impedance is infinite (open),while a point ZS is a point at which impedance is 0 (short). As shown inFIG. 3B, impedance Z10 of the first filter SAW1 seen from an observationpoint Ob10 which connects the third node C3 and the first filter SAW1 isrepresented by the impedance of the first filter SAW1 for a signal inthe stop band and is thus capacitive. Impedance Z21 of the first filterSAW1 seen from the end portion of the first switch SW1 connected to thefirst filter SAW1 is in the short state, which is close to the point ZS,because of the provision of the sixth switch SW6 that connects the thirdnode C3 and a ground point. Impedance Z31 of the first filter SAW1 seenfrom the common terminal P1 is not influenced by the impedance Z10 ofthe first filter SAW1 because the impedance Z21 is in the short state.That is, the impedance Z31 is represented by the impedance of the firstswitch SW1 that is turned OFF and is thus capacitive. The impedance Z31is closer to be infinite (open state) and is thus less likely to see theimpedance of the input/output terminal P2.

The impedance of the switch module 1 of the first embodiment will now beexplained below. FIG. 4A is an equivalent circuit diagram of the switchmodule 1 shown in FIG. 1. FIG. 4B is a Smith chart representing animpedance change on the signal path from the common terminal P1 to theinput/output terminal P2. As shown in FIG. 4B, impedance Z10 of thefirst filter SAW1 seen from the observation point Ob10 is similar tothat in the first comparative example. Impedance Z20 of the first filterSAW1 seen from the end portion of the first switch SW1 connected to thefirst filter SAW1 is not made to be in the short state without theprovision of a shunt-connected switch, and thus, the capacitiveimpedance Z10 is maintained. Impedance Z30 of the first filter SAW1 seenfrom the common terminal P1 is represented by the combined impedance ofthe capacitive impedance Z10 of the first filter SAW1 for a signal inthe stop band and the capacitive impedance of the first switch SW1 thatis turned OFF. The impedance Z30 is thus closer to the point ZO than theimpedance Z31 in the first comparative example and is in the open state.When the impedance Z30 is in the open state, the common terminal P1 andthe input/output terminal P2 are regarded as being disconnected fromeach other.

In the switch module 1 of the first embodiment, without ashunt-connected switch that connects a signal path and a ground point,impedance is not made to be in the short state in a range between thecommon terminal P1 and the input/output terminal P2. Instead, theimpedance of the input/output terminal P2 seen from the common terminalP1 is represented by the combined impedance of plural capacitiveimpedances, that is, the impedance of the first switch SW1 that isturned OFF and the impedance of the first filter SAW1 for a signal inthe stop band. The impedance of the input/output terminal P2 seen fromthe common terminal P1 is thus closer to the point ZO than that in thefirst comparative example. In FIGS. 3A through 4B, a comparison is madebetween an impedance change of the input/output terminal P2 seen fromthe common terminal P1 in the first embodiment and that in the firstcomparative example. Similarly, regarding an impedance change of thecommon terminal P1 seen from the input/output terminal P2, the impedanceof the common terminal P1 seen from the input/output terminal P2 in thefirst embodiment is also closer to the point ZO than that in the firstcomparative example.

A second comparative example in which the impedance of a filter isinductive (the imaginary part of the impedance is positive) will now bediscussed below with reference to FIGS. 5B through 6B. Then, the resultsof comparison between the insertion loss of the first embodiment andthat of the first and second comparative examples will be discussed withreference to FIG. 7. The second comparative example differs from thefirst embodiment in that the filter is inductive. The other points aresimilar to those of the first embodiment, and an explanation thereofwill thus be omitted.

FIG. 5A is a Smith chart representing simulation results of an impedancechange of a SAW filter in the first embodiment. FIG. 5B is a Smith chartrepresenting simulation results of an impedance change of a filter inthe second comparative example. In FIG. 5A, a curve S1 indicates animpedance change. The curve S1 included in a region PB indicates theimpedance for a signal in the pass band, while the curve S1 included ina region NPB1 indicates the impedance for a signal in the stop band. Thecurve S1 which is neither included in the region PB nor the region NPB1indicates the impedance of a signal in the transition band. As shown inFIG. 5A, the impedance of the SAW filter for a signal in the stop bandin the first embodiment changes within the region NPB1 where theimaginary part of the impedance is negative. That is, the impedance iscapacitive.

As shown in FIG. 5B, the impedance of the filter for a signal in thestop band in the second comparative example changes within a region NPB2where the imaginary part of the impedance is positive. That is, theimpedance is inductive.

FIG. 6A is an equivalent circuit diagram of a switch module 200 of thesecond comparative example. FIG. 6B is a Smith chart representing animpedance change on the signal path from the common terminal P1 to theinput/output terminal P2 in the switch module 200. The secondcomparative example differs from the first embodiment in that theimpedance of a filter for a signal in the stop band is inductive. Theother points are similar to those of the first embodiment, and anexplanation thereof will thus be omitted.

As shown in FIG. 6A, the switch module 200 includes first and secondfilters FLT1 and FLT2. The impedance of the first filter FLT1 for asignal in the stop band is inductive. The impedance of the second filterFLT2 for a signal in the stop band is also inductive.

As shown in FIG. 6B, impedance Z12 of the first filter FLT1 seen fromthe observation point Ob10 is represented by the impedance of the firstfilter FLT1 for a signal in the stop band and is thus inductive.Impedance Z22 of the first filter FLT1 seen from the end portion of thefirst switch SW1 connected to the first filter FLT1 is not made to be inthe short state without the provision of a shunt-connected switch, andthus, the inductive impedance Z12 is maintained. Impedance Z32 of thefirst filter FLT1 seen from the common terminal P1 is represented by thecombined impedance of the inductive impedance Z12 of the first filterFLT1 for a signal in the stop band and the capacitive impedance of thefirst switch SW1 that is turned OFF.

Combining of the capacitive impedance of the first switch SW1 that isturned OFF into the inductive impedance Z12 of the first filter FLT1causes the impedance Z12 moves to the point ZS and approaches the pointZO on the Smith chart. In the first comparative example, the capacitiveimpedance of the first switch SW1 that is turned OFF is combined intothe impedance in the short state close to the point ZS, thereby causingthe combined impedance to approach the point ZO. In the firstembodiment, the capacitive impedance of the first switch SW1 that isturned OFF is combined into the capacitive impedance of the first filterSAW1 for a signal in the stop band, thereby causing the combinedimpedance to approach the point ZO. The combined impedance in the secondcomparative example does not approach the point ZO as close as that inthe first comparative example and in the first embodiment. As a result,a greater insertion loss incurs in the second comparative example thanthat in the first comparative example and the first embodiment.

FIG. 7 is a graph representing the insertion loss of the switch module 1of the first embodiment and that of the switch modules 100 and 200 ofthe first and second comparative examples. In FIG. 7, the stop band ofthe first filter SAW1 shown in FIG. 1 and the pass band of the secondfilter SAW2 shown in FIG. 1 are both within a range of about 925 to 960MHz. A curve E1 represents the insertion loss of the switch module 1 ofthe first embodiment, and curves E10 and E20 respectively represent theinsertion loss of the switch modules 100 and 200 of the first and secondcomparative examples. In FIG. 7, the insertion loss is represented by anegative value, and as the absolute value of the insertion loss isgreater, a decrease in a signal from the input terminal to the outputterminal is greater. The magnitude of the absolute value represents themagnitude of the insertion loss. That is, in FIG. 7, a curve ispositioned above the other curves means that the absolute value of thiscurve is smaller than the absolute values of the other curves, and thus,the insertion loss represented by this curve is smaller than that by theother curves.

As shown in FIG. 7, in the frequency range of about 925 to 960 MHz, theinsertion loss of the switch module 1 of the first embodiment is smallerthan that of the switch modules 100 and 200 of the first and secondcomparative examples.

In the switch module 1 of the first embodiment, without ashunt-connected switch, impedance on a signal path from the commonterminal P1 to an input/output terminal via an associated switch that isturned OFF is not in the short state, but is in the open state due tothe combined capacitive impedance. As a result, it is possible to reducethe insertion loss of the switch module 1.

First Modified Example of First Embodiment

In the first embodiment, the pass band of the first filter SAW1 does notoverlap that of the second filter SAW2, and thus, the switch module 1does not include shunt-connected switches. However, if the pass band ofone filter overlaps that of another filter in a switch module, theprovision of shunt-connected switches is necessary. In a first modifiedexample of the first embodiment, the provision of shunt-connectedswitches is necessary because the pass band of one filter overlaps thatof another filter. In the first modified example, the configurations ofelements designated by like reference numerals of the first embodimentare similar to those of the first embodiment, and an explanation thereofwill thus be omitted.

FIG. 8 is a circuit diagram of a switch module 1A according to the firstmodified example of the first embodiment. The switch module 1A includesan input/output terminal P4, a third switch SW3, a third filter SAW3,which is a SAW filter, and fourth and fifth switches SW4 and SW5, whichare shunt-connected switches. The switch module 1A is a SP3T switchmodule. Alternatively, the switch module 1A may be a SPnT (n is four orgreater) switch module.

The pass band of the third filter SAW3 corresponds to a fourth frequencyband of an embodiment of the disclosure. The pass band of the secondfilter SAW2 and that of the third filter SAW3 overlap each other. Thepass band of the first filter SAW1 and that of the third filter SAW3 donot overlap each other.

On a path from the common terminal P1 to the input/output terminal P4,the third switch SW3 and the third filter SAW3 are connected in thisorder. The third switch SW3 switches between electrical connection anddisconnection between the common terminal P1 and the third filter SAW3.

The fourth switch SW4 switches between electrical connection anddisconnection between a ground point and a first node C1 between thesecond switch SW2 and the second filter SAW2. When the second switch SW2is turned OFF, the fourth switch SW4 is turned ON for a signal includedin both of the pass band of the second filter SAW2 and that of the thirdfilter SAW3.

The fifth switch SW5 switches between electrical connection anddisconnection between a ground point and a second node C2 between thethird switch SW3 and the third filter SAW3. When the third switch SW3 isturned OFF, the fifth switch SW5 is turned ON for a signal included inboth of the pass band of the second filter SAW2 and that of the thirdfilter SAW3.

In FIG. 8, the first switch SW1 is OFF; the second switch SW2 is ON andthe fourth switch SW4 is OFF; and the third switch SW3 is OFF and thefifth switch SW5 is ON. The ON/OFF states of the first through fifthswitches SW1 through SW5 are set in this manner when, for example, asignal MS2 included in both of the pass band of the second filter SAW2and that of the third filter SAW3 is input from the input/outputterminal P3 and is transmitted from the antenna connected to the commonterminal P1.

A signal LS2 may leak from the signal MS2 and be input into the signalpath to the input/output terminal P4. The signal LS2, which is a signalin the pass band of the third filter SAW3, may reach and pass throughthe third filter SAW3 and be output from the input/output terminal P4.

In the first modified example of the first embodiment, the signal LS2 ismostly shunted before reaching the third filter SAW3 because of the ONstate of the fifth switch SW5, which is a shunt-connected switch, and isnot influenced by the impedance of the third filter SAW3. Consequently,the signal LS2 which leaks from the signal MS2 is unlikely to be outputfrom the input/output terminal P4. In the switch module 1A, theisolation characteristics can be maintained even for a signal includedin the pass bands of plural filters.

In the first modified example of the first embodiment, it is possible toreduce the insertion loss of the switch module 1A and also to maintainisolation characteristics even for a signal included in the pass bandsof plural filters.

Second Modified Example of First Embodiment

In the first embodiment and the first modified example thereof, theswitch modules 1 and 1A include the common terminal P1 and pluralinput/output terminals connected to individual SAW filters. The singlecommon terminal P1 is used for all plural input/output terminals. In asecond modified example of the first embodiment, a switch moduleincludes two common terminals. The configuration between each SAW filterand the common terminal P1 in the second modified example is similar tothat of the first modified example of the first embodiment, and anexplanation thereof will thus be omitted.

FIG. 9 is a circuit diagram of a switch module 1B according to thesecond modified example of the first embodiment. As shown in FIG. 9, theswitch module 1B includes a common terminal P2B (second terminal),seventh, eighth, and ninth switches SW7, SW8, and SW9, and tenth andeleventh switches SW10 and SW11. The tenth and eleventh switches SW10and SW11 are shunt-connected switches.

The seventh switch SW7 switches between electrical connection anddisconnection between the common terminal P2B and the first filter SW1.The eighth switch SW8 switches between electrical connection anddisconnection between the common terminal P2B and the second filter SW2.The ninth switch SW9 switches between electrical connection anddisconnection between the common terminal P2B and the third filter SW3.The first filter SW1 is connected between the first and seventh switchesSW1 and SW7. The second filter SW2 is connected between the second andeighth switches SW2 and SW8. The third filter SW3 is connected betweenthe third and ninth switches SW3 and SW9.

When the seventh switch SW7 is turned OFF, the impedance of the seventhswitch SW7 is capacitive. The impedance of the first filter SAW1 seenfrom the end portion of the seventh switch SW7 connected to the firstfilter SAW1 is not in the short state, and the impedance of the firstfilter SAW1 seen from the common terminal P2B is in the open state.

The tenth switch SW10 switches between electrical connection anddisconnection between a ground point and a fourth node C4 between theeighth switch SW8 and the second filter SAW2. When the eighth switch SW8is turned OFF, the tenth switch SW10 is turned ON for a signal includedin both of the pass band of the second filter SAW2 and that of the thirdfilter SAW3.

The eleventh switch SW11 switches between electrical connection anddisconnection between a ground point and a fifth node C5 between theninth switch SW9 and the third filter SAW3. When the ninth switch SW9 isturned OFF, the eleventh switch SW11 is turned ON for a signal includedin both of the pass band of the second filter SAW2 and that of the thirdfilter SAW3.

In the second modified example of the first embodiment, it is possibleto reduce the insertion loss of the switch module 1B.

Second Embodiment

In the first embodiment, without the provision of a shunt-connectedswitch, a connection path from a series-connected switch to a SAW filteris not connected to a ground point so that the impedance from the commonterminal P1 to each input/output terminal can be made to be in the openstate. However, other measures may be taken to make the impedance fromthe common terminal P1 to each input/output terminal be in the openstate. Any measures may be taken not to connect a connection path from aseries-connected switch to a SAW filter to a ground point. In a secondembodiment, shunt-connected switches are provided, and regardless ofwhether series-connected switches are ON or OFF, the shunt-connectedswitches are turned OFF, so that a connection path from aseries-connected switch to a SAW filter is not connected to a groundpoint. In the second embodiment, the configurations of elementsdesignated by like reference numerals of the first embodiment aresimilar to those of the first embodiment, and an explanation thereofwill thus be omitted.

FIG. 10 is a circuit diagram of a switch module 2 according to thesecond embodiment. As shown in FIG. 10, the switch module 2 includesfourth and sixth switches SW4 and SW6 as shunt-connected switches.

The fourth switch SW4 switches between electrical connection anddisconnection between a ground point and a first node C1 between thesecond switch SW2 and the second filter SAW2. The fourth switch SW4 isturned OFF regardless of whether the second switch SW2 is ON or OFF.

The sixth switch SW6 switches between electrical connection anddisconnection between a ground point and a third node C3 between thefirst switch SW1 and the first filter SAW1. The sixth switch SW6 isturned OFF regardless of whether the first switch SW1 is ON or OFF.

In the switch module 2 of the second embodiment, the shunt-connectedswitches are turned OFF regardless of the first and second switches areON or OFF, so that the impedance of the first filter SAW1 seen from theend portion of the first switch SW1 connected to the first filter SAW1will not be made to be in the short state. With this configuration, theimpedance on a signal path from the common terminal P1 to aninput/output terminal via an associated switch that is turned OFF is notin the short state, but is in the open state. As a result, it ispossible to reduce the insertion loss of the switch module 2, as in theswitch module 1 of the first embodiment.

First Modified Example of Second Embodiment

In a first modified example of the second embodiment, a switch module 2A(FIG. 11) includes a shunt-connected switch because the pass bands oftwo filters in the switch module 2A overlap each other, as in the firstmodified example of the first embodiment. The first modified example ofthe second embodiment differs from the first modified example of thefirst embodiment in that the sixth switch SW6 discussed in the secondembodiment is connected between a ground point and the third node C3between the first switch SW1 and the first filter SAW1. Theconfigurations of the other elements are similar to those of the firstmodified example of the first embodiment, and an explanation thereofwill thus be omitted.

In the first modified example of the second embodiment, it is possibleto reduce the insertion loss of the switch module 2A, as in the firstmodified example of the first embodiment.

Second Modified Example of Second Embodiment

In a second modified example of the second embodiment, a switch module2B (FIG. 12) includes two common terminals, as in the second modifiedexample of the first embodiment. The second modified example of thesecond embodiment differs from the second modified example of the firstembodiment in that a sixth switch SW6 is connected between a groundpoint and a third node C3 between the first switch SW1 and the firstfilter SAW1 and that a twelfth switch SW12 is connected between a groundpoint and a sixth node C6 between the seventh switch SW7 and the firstfilter SAW1. The configurations of the other elements are similar tothose of the second modified example of the first embodiment, and anexplanation thereof will thus be omitted. The sixth switch SW6 issimilar to that of the second embodiment, and an explanation thereofwill thus be omitted.

As shown in FIG. 12, the switch module 2B includes the twelfth switchSW12. The twelfth switch SW12 switches between electrical connection anddisconnection between a ground point and the sixth node C6 between theseventh switch SW7 and the first filter SAW1. The twelfth switch SW12 isturned OFF regardless of whether the seventh switch SW7 is ON or OFF.The impedance of the first filter SAW1 seen from the end portion of theseventh switch SW7 connected to the first filter SAW1 is not in theshort state, and the impedance of the first filter SAW1 seen from thecommon terminal P2B is in the open state.

In the second modified example of the second embodiment, it is possibleto reduce the insertion loss of the switch module 2B, as in the secondmodified example of the first embodiment.

Third Embodiment

In the first and second embodiments, SPDT and SP3T switch modules areused as examples of a SPnT switch module according to an embodiment ofthe disclosure. However, the disclosure may be applicable to other typesof SPnT switch modules. In a third embodiment, a SP6T switch module isused as an example of the SPnT switch module.

FIG. 13 is a circuit diagram of a switch module 3 according to the thirdembodiment. As shown in FIG. 13, the switch module 3 includes a commonterminal P1, first, second, and third switches SW1, SW2, and SW3,input/output terminals P12, P13, P22, P23, P32, and P33, and duplexersDUP1, DUP2, and DUP3.

The first switch SW1 switches between electrical connection anddisconnection between the duplexer DUP1 and the common terminal P1. Thesecond switch SW2 switches between electrical connection anddisconnection between the duplexer DUP2 and the common terminal P1. Thethird switch SW3 switches between electrical connection anddisconnection between the duplexer DUP3 and the common terminal P1.

The duplexer DUP1 includes a transmit circuit Tx1 and a receive circuitRx1. The transmit circuit Tx1 is connected between the common terminalP1 and the input/output terminal P12. The transmit circuit Tx1 includesa SAW filter (not shown). The receive circuit Rx1 is connected betweenthe common terminal P1 and the input/output terminal P13. The receivecircuit Rx1 includes a SAW filter (not shown).

The duplexer DUP2 includes a transmit circuit Tx2 and a receive circuitRx2. The transmit circuit Tx2 is connected between the common terminalP1 and the input/output terminal P22. The transmit circuit Tx2 includesa SAW filter (not shown). The receive circuit Rx2 is connected betweenthe common terminal P1 and the input/output terminal P23. The receivecircuit Rx2 includes a SAW filter (not shown).

The duplexer DUP3 includes a transmit circuit Tx3 and a receive circuitRx3. The transmit circuit Tx3 is connected between the common terminalP1 and the input/output terminal P32. The transmit circuit Tx3 includesa SAW filter (not shown). The receive circuit Rx3 is connected betweenthe common terminal P1 and the input/output terminal P33. The receivecircuit Rx3 includes a SAW filter (not shown).

The common terminal P1, the input/output terminals P12, P22, and P32,the first, second, and third switches SW1, SW2, and SW3, and thetransmit circuits Tx1, Tx2, and Tx3 form a SP3T switch module accordingto an embodiment of the disclosure.

The common terminal P1, the input/output terminals P13, P23, and P33,the first, second, and third switches SW1, SW2, and SW3, and the receivecircuits Rx1, Rx2, and Rx3 form a SP3T switch module according to anembodiment of the disclosure.

The switch module 3 includes two SP3T switch modules according to anembodiment.

In the third embodiment, an antenna ANT is connected to the commonterminal P1. A power amplifier (PA) is connected to the input/outputterminals P12, P22, and P32. A low noise amplifier (LNA) is connected tothe input/output terminals P13, P23, and P33. The circuit diagram shownin FIG. 13 may be used for a transmit-and-receive circuit of a mobileterminal, such as a smartphone.

Instead of the duplexers DUP1 through DUP3, triplexers may be used. Inthis case, a switch module according to the third embodiment includesthree SP3T switch modules and serves as a SP9T switch module.

In the third embodiment, it is possible to reduce the insertion loss ofthe switch module 3, as in the first and second embodiments.

The above-described embodiments may be combined in a suitable mannerwithin a technically possible range. The disclosed embodiments are onlyexamples and are not intended to be exhaustive or to limit the inventionto the precise forms disclosed.

While preferred embodiments of the invention have been described above,it is to be understood that variations and modifications will beapparent to those skilled in the art without departing from the scopeand spirit of the invention. The scope of the invention, therefore, isto be determined solely by the following claims.

What is claimed is:
 1. A switch module comprising: a first terminal; afirst filter that allows a signal in a first frequency band to passthrough the first filter and stops a signal in a second frequency bandfrom passing through the first filter; a first switch that controls anelectrical connection between the first terminal and the first filter,the first terminal and the first filter being connected when the firstswitch is ON and the first terminal and the first filter beingdisconnected when the first switch is OFF; a second filter that allows asignal in a third frequency band to pass through the second filter, thethird frequency band being included in the second frequency band; and asecond switch that controls an electrical connection between the firstterminal and the second filter, the first terminal and the second filterbeing connected when the second switch is ON and the first terminal andthe second filter being disconnected when the second switch is OFF,wherein the first filter has a capacitive impedance for a signal in thesecond frequency band, and when the first switch is OFF: the firstswitch has a capacitive impedance, an impedance of the first filter asseen from the first switch is not shorted, and an impedance of the firstfilter as seen from the first terminal is open.
 2. The switch moduleaccording to claim 1, further comprising: a third filter; a third switchthat controls an electrical connection between the first terminal andthe third filter, the first terminal and the third filter beingconnected when the third switch is ON and the first terminal and thefirst filter being disconnected when the third switch is OFF; a fourthswitch that controls an electrical connection between ground and a firstnode between the second switch and the second filter, the first nodebeing connected to ground when the fourth switch is ON and the firstnode being disconnected from ground when the fourth switch is OFF; and afifth switch that controls an electrical connection between ground and asecond node between the third switch and the third filter, the secondnode being connected to ground when the fifth switch is ON and thesecond node being disconnected from ground when the fifth switch is OFF,wherein the third filter allows a signal in a fourth frequency band topass through the third filter, the fourth frequency band overlapping thethird frequency band, when the second switch is OFF, the fourth switchis ON for a signal of a frequency included in both the third frequencyband and the fourth frequency band, and when the third switch is OFF,the fifth switch is ON for the signal of the frequency included in boththe third frequency band and the fourth frequency band.
 3. The switchmodule according to claim 1, wherein a connection path between the firstswitch and the first filter is not connected to ground.
 4. The switchmodule according to claim 1, further comprising: a sixth switch thatcontrols an electrical connection between ground point and a third nodebetween the first switch and the first filter, the third node beingconnected to ground when the sixth switch is ON and the third node beingdisconnected from ground when the sixth switch is OFF, wherein the sixthswitch is OFF regardless of whether the first switch is ON or OFF. 5.The switch module according to claim 2, further comprising: a sixthswitch that controls an electrical connection between ground point and athird node between the first switch and the first filter, the third nodebeing connected to ground when the sixth switch is ON and the third nodebeing disconnected from ground when the sixth switch is OFF, wherein thesixth switch is OFF regardless of whether the first switch is ON or OFF.6. The switch module according to claim 1, wherein the first filter andthe second filter are surface acoustic wave filters.
 7. The switchmodule according to claim 1, further comprising: a second terminal; aseventh switch that controls an electrical connection between the secondterminal and the first filter, the second terminal and the first filterbeing connected when the seventh switch is ON and the second terminaland the first filter being disconnected when the seventh switch is OFF;and an eighth switch that controls an electrical connection between thesecond terminal and the second filter, the second terminal and thesecond filter being connected when the eighth switch is ON and thesecond terminal and the second filter being disconnected when the eighthswitch is OFF, wherein the first filter is connected between the firstswitch and the seventh switch, the second filter is connected betweenthe second switch and the eighth switch, and when the seventh switch isOFF: the seventh switch has a capacitive impedance, an impedance of thefirst filter as seen from the seventh switch is not shorted, and animpedance of the first filter as seen from the second terminal is open.8. The switch module according to claim 7, further comprising: a thirdfilter that allows a signal in a fourth frequency band to pass throughthe third filter, the fourth frequency band overlapping the thirdfrequency band; a third switch that controls an electrical connectionbetween the first terminal and the third filter, the first terminal andthe third filter being connected when the third switch is ON and thefirst terminal and the third filter being disconnected when the thirdswitch is OFF; a fourth switch that controls an electrical connectionbetween ground and a first node between the second switch and the secondfilter, the first node being connected to ground when the fourth switchis ON and the first node being disconnected from ground when the fourthswitch is OFF; a fifth switch that controls an electrical connectionbetween ground and a second node between the third switch and the thirdfilter, the second node being connected to ground when the fifth switchis ON and the second node being disconnected from ground when the fifthswitch is OFF; a ninth switch that controls an electrical connectionbetween the second terminal and the third filter, the second terminaland the third filter being connected when the ninth switch is ON and thesecond terminal and the third filter being disconnected when the ninthswitch is OFF; a tenth switch that controls an electrical connectionbetween ground and a fourth node between the eighth switch and thesecond filter, the fourth node being connected to ground when the tenthswitch is ON and the fourth node being disconnected from ground when thetenth switch is OFF; and an eleventh switch that controls an electricalconnection between ground and a fifth node between the ninth switch andthe third filter, the fifth node being connected to ground when theeleventh switch is ON and the fifth node being disconnected from groundwhen the eleventh switch is OFF, wherein the third filter is connectedbetween the third and ninth switches, when the second switch is OFF, thefourth switch is ON for a signal of a frequency included in both thethird frequency band and the fourth frequency band, when the thirdswitch is OFF, the fifth switch is ON for the signal of the frequencyincluded in both the third frequency band and the fourth frequency band,when the eighth switch is OFF, the tenth switch is ON for the signal ofthe frequency included in both the third frequency band and the fourthfrequency band, and when the ninth switch is OFF, the eleventh switch isON for the signal of the frequency included in both the third frequencyband and the fourth frequency band.
 9. The switch module according toclaim 7, wherein a connection path between the seventh switch and thefirst filter is not connected to ground.
 10. The switch module accordingto claim 7, further comprising: a twelfth switch that controls anelectrical connection between ground and a sixth node between theseventh switch and the first filter, the sixth node being connected toground when the twelfth switch is ON and the sixth node beingdisconnected from ground when the twelfth switch is OFF, wherein thetwelfth switch is OFF regardless of whether the seventh switch is ON orOFF.
 11. The switch module according to claim 1, further comprising: afourth switch that controls an electrical connection between ground anda first node between the second switch and the second filter, the firstnode being connected to ground when the fourth switch is ON and thefirst node being disconnected from ground when the fourth switch is OFF;and a sixth switch that controls an electrical connection between groundpoint and a third node between the first switch and the first filter,the third node being connected to ground when the sixth switch is ON andthe third node being disconnected from ground when the sixth switch isOFF.
 12. The switch module according to claim 2, wherein a connectionpath between the first switch and the first filter is not connected toground.
 13. The switch module according to claim 8, wherein a connectionpath between the seventh switch and the first filter is not connected toground.
 14. The switch module according to claim 8, further comprising:a twelfth switch that controls an electrical connection between groundand a sixth node between the seventh switch and the first filter, thesixth node being connected to ground when the twelfth switch is ON andthe sixth node being disconnected from ground when the twelfth switch isOFF, wherein the twelfth switch is OFF regardless of whether the seventhswitch is ON or OFF.